For the purpose of chip-to-chip, board-to-board, and system-to-system communications over high bandwidth distance links, the use of optical interconnection networks as replacements for their electrical counterparts is steadily growing. For links of high bit rate and distance (for example, greater than 10 meter-gigabits per second), optical transmission technology is increasingly providing better cost and performance versus electrical transmission technology. An additional advantage of optical transmission technology is that it can provide multi-point links without significant reduction in performance.
Multi-drop and multi-point links are particularly useful in computer and communications systems with many integrated processing units, such as for Symmetric Multi-processor (SMP) buses, memory buses, and I/O buses in high-end systems, since such links allow close coupling between multiple different devices without multiple separate point-to-point links. For example, the “broadcast-and-snoop” protocols used for assuring coherence between processor caches could be most effectively implemented over a parallel bus of many multi-point links, if they could be made to operate at sufficiently high bit rate. High frequency electrical signaling limitations limit practical operation of multi-point links to a bit rate of roughly 500 Megabits/sec, which is increasingly insufficient for modem computer systems, so implementation of the broadcast functionality is typically implemented with a cascaded network of point-to-point links, greatly increasing system cost, complexity, and power consumption.
In the prior art, the optical equivalent for the multi-point link is an “optical star coupler”. In an optical star coupler, optical data streams from each of the inputs are combined in the star coupler and physically distributed to all of the outputs, so that each input can (with appropriate arbitration for access) broadcast data to all of the outputs. These optical star coupler networks are created through the careful integration of devices that perform functions such as beam-splitting, “fan-in”, “fan-out”, and coupling. Optical star couplers are common components in the area of optical interconnection networks for transmitting each of multiple input signals to all of multiple destinations, thus connecting each of N inputs to all of N outputs simultaneously, N is an integer greater than zero.
Prior art star couplers are characterized by numerous disadvantages that make prior art star couplers unsatisfactory for use in computer broadcast bus interconnection networks. For example, prior art star couplers exhibit limited efficiency in coupling the input and output waveguides; exhibit limited efficiency in distributing input power from the input to the output waveguides; are not readily scalable for more complex applications; and are limited to a single-plane geometry. Most particularly, prior art star couplers do not allow integrated construction of multiple independent star coupler functions for multiple independent sets of links, as is required for multi-link computer bus interconnection networks.